Sheet metal stretch forming apparatus and method



Jan. 24, 1967 SHEET METAL STRETCH FORMING APPARATUS AND METHOD 3 Sheets-Sheet 1 Filed June 15, 1964 L. R. GRAY Jan. 24, 1967 SHEET METAL STRETCH FORMING APPARATUS AND METHOD Filed June 15, 1964 3 Sheets-Sheet 2 INVENTOR. [AA/00m Z xe/ Ar azgA/l Jan. 24, 1967 L. R. GRAY 3,299,633

SHEET METAL STRETCH FORMING APPARATUS AND METHOD Filed June 15, 1964 3 Sheets-Sheet 3 H 1 5!. I INVENTOR.

l/i/vam/ 2 @QA United States Patent 3,299,688 SHEET METAL STRETCH FORMING APPARATUS AND METHOD Landon R. Gray, 24701 Crenshaw Blvd., Torrance, Calif. 90505 Filed June 15, 1964, Ser. No. 374,944 8 Claims. (Cl. 72-296) This invention lies in the field of stretch forming sheet metal over dies having surfaces of double curvature, and it is directed particularly to a stretching complex made up of discrete stretch units adapted to work in predetermined relation to each other, and to a method of stretch forming sheet metal over dies of double curvature with a predetermined percentage of elongation in all portions of the blank by the use of the novel stretching complex. In practically all cases it is desired to produce the same percentage of elongation throughout the blank, but the method and apparatus disclosed are capable of producing predetermined non-uniform percentages of elongation for special purposes.

The general art of stretch forming dates back several decades. One of the early types of sheet metal forming machines which went into general use included a base, a die supporting ram or jack, and a pair of jaws. The ram was usually vertically movable and the die was mounted on top of it. In operation a sheet of thin, easily bendable metal was placed over the die and resiliently bent so that a pair of opposed edges could be inserted in the jaws. The jaws, which were stationarily mounted, were tightened to grip the edges or margins of the sheet, and the ram was then operated to force the die upward into the sheet until the latter was stressed beyond the yield point and took a permanent set.

The jaws on these machines were straight and rigid and gripped the sheet margin in a straight line. Thus the entire machine was very simple and worked very well when the curvature of the die laterally of the direction of stretch was relatively slight. When the lateral curvature was substantially increased, the excess portion of the sheet was correspondingly increased to provide for the transition from the curvature at the edge of the die to the straight line at the gripper jaw. This resulted in undesirable trim waste. The first solution was to supply sets of curved rigid jaws to match the dies, and this reduced the waste but increased the amount of equipment needed.

At a later period machines were developed primarily for forming rolled or extruded elongate members commonly called structural sections. These machines utilized movable jaws and usually movable dies on rams such as in the prior machines. With the movable jaws, the elongate sections could be pre-stressed or stretched beyond the elastic limit in a straight line and then wrapped around the face of the die by relative movement of the die and the aws.

The same general principles were then applied to sheet machines, and very large sheets were formed in this way, still utilizing rigid fixed shape jaws. A refinement was eventually added which consisted in utilizing a large, strong, rigid jaw base to which a series of individual jaws were adjustably attached. With this arrangement, the individual jaws could be adjusted to match the curvature of any particular die. They were then locked in adjusted position and acted as a single rigid jaw of a particular curvature for use with that particular die. One example of such a jaw is illustrated in US. Patent No. 2,609,860. laws of this type were incorporated in some large machines in which the die was fixedly mounted and the jaws were movable away from the die at each of two opposing sides.

3,299,688 Patented Jan. 24, 1967 While all of the machine and jaw combinations described above can form parts very satisfactorily in the sense that the parts will take and retain the desired shape, they all have one serious drawback when applied to the stretch forming of sheet stock, or plate stock if the machine is powerful enough to handle it. The die surfaces are fundamentally convex, or of double curvature, even if some areas may be considered to be concave when taken locally. The jaws are rigid, unchanging in shape during a forming operation, and there is the same amount of relative movement between each portion of a jaw and its respective portion of the die.

The sheet is initially flat and is bent or curved in only one direction when first attached to the jaws. It might be said to be in the form of a partial cylinder with an axis eX- tending laterally of the direction of stretch. Assuming for illustration that the die surface is substantially spherical, the sheet will first contact the high point and remain spaced from the low points. It is obvious that there will be a substantial difference in the lengths of the surface paths over the high and low points or portions of the die face parallel to the direction of stretch. By the time the sheet even contacts the low point paths it has been stretched considerably along the high point path and will continue to be stretched even more by the time the sheet has passed the yield point on the low point paths. This results in excessive and undesirable thinning. If the resultant factor of safety is too low, then thicker stock must be used. This in turn produces either a weight penalty if the extra thickness is retained throughout, or a cost penalty if the extra thickness is removed by machining, etching, or other means.

The present invention completely overcomes the dithculties mentioned above. Considering one broad aspect of the method, it consists in pulling lengthwise each increment of width of the sheet, which term is to be understood as including stock which is heavy enough to be called plate, until it has been elongated by the same predetermined percentage of its original length and then discontinuing the pulling or stretching of that particular increment. The extent of lengthwise movement of each increment of margin of the sheet is of course dependent on the length of that increment of the sheet and also on the length of the surface path of the corresponding die increment in the direction of stretch.

There are several ways of accomplishing the above but the two presently preferred systems are (l) to pull all increments at a constant rate of movement and stop the pull on each increment as its elongation reaches the predetermined percentage, and (2) to pull all increments at various predetermined rates so that in the same period of time all increments will have elongated the predetermined percentage of their original lengths. In another broad aspect of the method, the amount of stretching movement of the increments is predetermined to give a non-uniform percentage of elongation of the increments to produce a predetermined and desired variation in thickness of different portions of the finished article.

In one preferred type of apparatus for carrying out the invention an elongate base is provided and a die is mounted on an intermediate portion of the base. The die may be stationarily mounted or it may be carried on a ram for movement toward and away from the base. Such movement is normally vertical since large machines of this type are usually much more convenient to service and operate in such attitude. However it is to be understood that such terms as vertical or horizontal, upper or lower, etc. are used for convenience of description and claiming, and are not to be taken as limitative of the manner in which the invention can be carried out.

At each of two opposing sides of the die a stretch-complex is mounted on the base. Each complex is composed of a plurality of discrete, independent, separately operable stretch units which in turn basically consist of an anchor, a jack device movably connected at one end to the anchor, and a gripper movably connected to the other end of the jack device. The latter may be mechanical or otherwise but is preferably a hydraulic piston and cylinder unit. The gripper preferably has a hydraulically actuated jaw. The anchors are independently movable toward and away from the die along tracks incorporated in the base and have individual actuators which in their present form comprise electric motors, reduction gearing, threaded shafts and mating nuts. In apparatus for limited utility with special purpose work, adjusting means for the anchors may be omitted.

In practicing the method described above, the stretch units are moved to positions in which the jaws can grip the margin of the blank. The jaws are arranged in juxtaposition and are preferably almost touching so that the stretching load will be applied to all increments of width of the blank. The stretch units are preferably adjusted so that as stretching is being completed the axis of each unit will be substantially tangent to the surface of the adjacent part of the die so that the stretching force will be along that tangent. The jack devices of the stretch unit are now actuated to retract them. The operator can control them individually or they can be controlled by a tape programmer which has been worked out by experiment, or the operation can be under the command of the positive position control system disclosed and claimed in my Patent No. 2,824,594, dated February 25, 1958.

Whether the apparatus is operated manually or automatically, each jack retracts until the increment gripped by its jaw has elongated by the desired percentage and then it stops. Because of the generally convex or dome shaped die face, the jaws are retracted different distances.

Since the anchors, jack devices, and grippers are all movably connected, the gripper jaws are free to turn in accordance with the developing curvature of the margin of the blank and apply a more even stretching force to all of the blank. In most instances a stretching complex operating as above described is necessary at each of two opposing sides of a die. However, in cases where the die runs out at one side to a practically planar form, a single, straight rigid jaw may be used at that side and may not even move relative to the die during the stretch forming operation.

Various other advantages and features of novelty will become apparent as the description proceeds in conjunction with the accompanying drawings, in which:

FIGURE 1 is a perspective View of one form of the apparatus of this invention with portions broken away to show details;

FIGURE 2 is a side elevational view of the apparatus of FIGURE 1 partly in section;

FIGURE 3 is a sectional view taken on line 3-3 of FIGURE 2;

FIGURE 4 is a sectional view taken on line 44 of FIGURE 2; and

FIGURE 5 is a top view, generally in plan, taken in the direction of line 55 of FIGURE 2.

The presently preferred form of the apparatus of the invention is illustrated in FIGURES l and 2. The base is set in a deep pit 12 so that the die 14 will be at a convenient location above the floor 16 for loading and unloading and for operation. The base is made up generally of elongate structural members 18 secured together with other structural members not shown and fasteners 20 to make a strong rigid support for the balance of the machine. Members 18 as shown are channel members and are arranged in pairs with the channels opening toward each other to form undercut or T-slot tracks 36 for a purpose to be disclosed.

Die support 22 is mounted directly on members 18 and may be of any construction such as built-up steelwork or solid or hollow concrete or building blocks strong enough to take the very heavy load imposed on it. The die 14 itself is a rather simple but relatively light and effective open construction including a skin section 24 having a convex or double curvature surface bearing the desired shape of the finished part, and structural members 26 to strengthen and stiffen the skin.

The sheet metal blank 28 is laid over die 14 in position to be gripped and shaped by stretching over the die. The blank is limited in thickness only by the capability of the machine and may Well fit the definition or designation of plate. It is not limited to any particular shape but is here shown as being initially generally rectangular, which is true of most blanks. Two marginal edges, one of which is indicated at 30, extend over what might be called the fore and aft ends of the die enough to be received in gripping jaws.

As shown in FIGURES 1 and 2 the blank is practically finish formed but FIGURE 3 indicates the way in which it confronts the die surface at the beginning or at an intermediate point in the forming. It cannot fit both fore and aft and laterally until it has been stretched. Since it is first conformed to the fore and aft curvature it does not touch the die face at the sides, but rather leaves gaps indicated at 32 in FIGURE 3. It will be observed that the middle increment of the blank will be substantially stretched before the side increments even contact the die. This is the major problem solved by the present invention.

A stretching complex 34 is normally located at each sideor each end--of the die and its support, although as mentioned above there may be circumstances in which only one complex is necessary and a simple rigid gripping jaw may be used at the other side. Each complex is composed of a battery or group of discrete, independent, and substantially identical stretch units 38, only one of which will be described. One unit comprises an anchor 40 with guide grooves 42 to slidably engage tracks 36, trunnion pin 44, clevis yoke 46, cylinder 48 pivoted to the clevis yoke and provided with piston rod 50. The latter carries a yoke 52 which pivotally supports a gripper 54. The specific details of the gripper are not set forth herein because they do not form a part of the present invention. However they are disclosed in my prior joint patent to L. R. Gray et al., No. 2,835,947, dated May 27, 1958.

Each anchor 40 is adapted to slide in its track 36 toward and away from the die. The interengagement with the T-slot track 36 takes the vertical component of stretching loads on the unit. A screw threaded rod or lead screw 56 is carried for rotation by supports 58 and 60, but translation is not permitted. The lead screw is driven by servo motor 62 through reduction drive 64 and has a threaded connection with the anchor so that as it rotates it translates its associated anchor. The interconnection between them takes the horizontal component of stretching loads on the unit.

Machines of this type are generally quite large and the one illustrated is the largest of its kind. Each of the six cylinders seen in FIGURE 1 generates a pull or stretching force of five hundred tons so that the machine as shown has a capacity of three thousand tons. Each stretch unit weighs thousands of pounds and hence cannot be handled manually. To raise the units to the desired angle or attitude and support them there, each is provided with a chain 66 secured at one end to its respective cylinder. The chain passes over a pulley, not shown, carried by standard 68 and its second end is secured to piston rod 70 of servo motor 72. The servo motor, which is connected into the hydraulic system, raises the cylinders as desired and holds them in position.

Each gripper 54 is a massive, strong block of metal with a yoke comprising fixed jaws 74 and 76 spaced to receive the marginal edge of a blank. A movable jaw 78 is carried by jaw 76 to cooperate with jaw 74 in tightly gripping the margin, and is powered by hydraulic cylinders, not shown, in jaw 76. A pair of small hydraulic servos 80 extend between the shoulder of each yoke 52 and the rear wall of the corresponding gripper and are actuated to align the gripper with its piston rod as desired. The grippers, weighing hundreds of pounds each, are far too heavy to be adjusted manually.

While the grippers and the balance of their respective stretch units are individually adjustable and operable, the grippers themselves are held in loosely assembled relation to assist in aligning their yoke openings to receive a blank to be processed. A very simple arrangement is shown for illustrative purposes only in FIGURES 4 and 5. The confronting faces of each pair of grippers are provided with bosses 82 having ball-socket faces and passages therethrough. Between each pair of sockets is mounted a ball 84, also having a passage therethrough. All of the grippers have passages therethrough in alignment with the balls and sockets, and a flexible cable 86 extends through all of the passages. The cable connects at one end to an anchor 88 and at the other end to the piston of a servo cylinder 90.

In addition to the individual supports 66-72 for the cylinders, the grippers may be supported by an overhead whifiletree beam 92 connected to a hoist not shown. Cables or rods 94 are pivotally connected to the ends of beam 92 and also to bosses 96 on the two end grippers. When it is desired to load the rather straight margin of a new blank into the assembly of grippers, the beam is raised to take their weight. Cylinder 90 is actuated to tension cable 86 so the grippers will pivot around the ball socket joints and will not separate. The assembly would normally sag to an upwardly concave attitude but this is prevented by the provision of abutments or stop blocks 98 welded on the upper sides of the grippers. They are so located that with the cable tensioned and the load on beam 92 the assembly of grippers will be a straight horizontal line. When the grippers have been actuated to firmly grip the margin of the blank, beam 92 is lowered and may even be disconnected. Servo cylinder 90 is actuated to reduce or eliminate the tension in cable 86 and the grippers will now be free to act independently of each other during the stretch forming operation.

Because of the fact that the individual anchors 40 can be adjusted horizontally toward and away from the die it is possible to set each stretch unit in such position that the stretching force acting along the axis of the stretch unit cylinder will be in a line substantially tangent to the surface of the die at the die margin. With prior machines, even if the jaws could be adjusted to a curve, the stretching movement of the entire jaw was in a single direction which had to be a compromise to take care of the steepest slope. This meant pulling the blank around a corner on the shallower slopes, Which is obviously undesirable.

With the apparatus and arrangement described above it will be seen that it is now possible to stretch form a blank over a die having a high degree of double curvature and still obtain optimum results. The two most important factors are pulling along the tangent line for each increment of width and retracting the stretch units to different extents in order to stretch the various increments of width of the blank to the same extents; that is, to achieve the same percentage of elongation throughout the finished article. Moreover, in those instances where local variations in final thickness are desired for special reasons the machine can be operated to produce these results, and the greatest thinning can be accomplished at either the high points or the low points. FIGURE 5 illustrates the normal case where uniform elongation is desired. It will be seen that the center stretch units have retracted the least and the outermost units have retracted the most and have pulled the margin 30 of the blank forward and somewhat in toward the center line. As stated above, the stretch units can operate at the same rate, with the center units stopping first and the other units in order, or the units can start together, retract at different rates, and stop together.

All of the actions of the machine are under the command of the operator at the control console 100. Hydraulic lines such as 102 run to all of the hydraulic devices of the apparatus but they have been omitted for clarity of illustration. The operator can control all of the operations manually and separately, or he can control a limited number of them, using a system such as disclosed in my prior Patent No. 2,824,594 mentioned above. When a large number of identical parts are to be formed, the console may also make use of a punched tape programmer.

The machine illustrated in the drawings utilizes six grippers in each stretching complex. A smaller or larger number may be used, depending on the severity of curvature of the die and on other factors. Various other changes and modifications may also be made without departing from the spirit of the invention and it is intended that all such changes and modifications shall be embraced within the scope of the following claims.

I claim:

1. Apparatus for stretch forming a sheet metal blank to a concavo-convex shape, comprising: an elongate base; a die mounted on an intermediate portion of said base and having an upwardly directed convex forming face; a stretching complex mounted on each end portion of said base at opposed sides of said die and adapted to grip opposed marginal edges of a metal blank to stretch and form it on the forming face of said die; each complex including a plurality of discrete, independent, elongate stretch units; each stretch unit including an independent anchor, a jack device pivotally connected at one end to said anchor for vertical and horizontal swinging, and a gripper pivotally connected to the other end of the jack device for rotation and. lateral swinging and provided with a jaw to grip a portion of the margin of a metal blank; a plurality of elongate, substantially parallel track means at each end of said base and extending generally longitudinally of the base; said anchors having portions slidably interengaging with said track means; and separate servo actuators carried by the base and connected to respective anchors to independently move them 1ongitudinally on said tracks toward and away from said die to provide varied predetermined settings of the stretch units with respect to each other and to the die and blank; the jack devices of the various stretch units being individually operable to control their total retraction and the consequent percentage of elongation of various portions of the blank in accordance with a predetermined pattern.

2. Apparatus as claimed in claim 1; the relative rate of retraction of the various jack units being controllable to provide different patterns of stretch forming operation.

3. Apparatus as claimed in claim 1; said servo actuators including elongate, threaded, rotatable shafts extending longitudinally of the base and threadedly engaging said anchors, and serve motors to selectively rotate said shafts to vary the horizontal positions of said anchors; the interengagement of the anchors and the track means on said base taking up the vertical component of the stretch forming load, and the interengagement of the anchors and the shafts taking up the horizontal component of the stretch forming load.

4. Apparatus as claimed in claim 1; and individual servo mechanisms connected to each stretch unit to tilt it about its anchor and support it at the proper angle for the stretch forming operation.

5. Apparatus for stretch forming a sheet metal blank to a concavo-convex shape, comprising: a base; a die mounted on said base and having a convex forming face; a stretching complex mounted on said base at each of two opposed sides of said die and adapted to grip opposed marginal edges of a metal blank to stretch and form it on the forming face of said die; each complex including a plurality of discrete, independent, elongate stretch units;

7 each stretch unit including an independent anchor, a jack device movably connected at one end to said anchor for vertical and horizontal swinging, and a gripper movably connected to the other end of the jack device for rotation and lateral swinging; said gripper having an outwardly opening yoke to receive and grip a portion of the margin of a metal blank; said anchor being independently adjustably mounted on said base for horizontal movement toward and away from said die so that the stretch units may be initially positioned with their axes substantially tangent to the surface of that portion of the die adjacent their respective yokes and with the yokes gripping their respective portions of the margin of the blank; each jack device being independently retractable to the extent necessary to produce a uniform percentage of elongation in all portions of the blank.

6. Apparatus as claimed in claim 5; all of said jack devices operating at a uniform rate of retraction and stopping sequentially as each of them accomplishes the predetermined percentage of elongation of its respective portion of the blank.

7. Apparatus as claimed in claim 5; all of said jack devices beginning and ending their operation at the same times; each jack device retracting at a preselected rate proportional to the physical distance it must travel in the allotted time to accomplish the predetermined percentage of elongation in its respective portion of the blank.

8. Apparatus as claimed in claim 5; separate servo actuators being provided to move each anchor; each actuator being independently operable to change the position of its anchor before, during, or after the stretch forming operation.

References Cited by the Examiner UNITED STATES PATENTS 2,218,503 10/1940 Brooks et al. 72302 2,437,092 3/ 1948 Greene et al. 72395 2,692,633 10/1954 Green 72-302 2,961,028 11/1960 Bath 72297 3,082,809 3/1963 Petsch et a1 72-302 CHARLES W. LANHAM, Primary Examiner.

RICHARD J. HERBST, Examiner.

L. A. LARSON, Assistant Examiner. 

1. APPARATUS FOR STRETCH FORMING A SHEET METAL BLANK TO A CONCAVO-CONVEX SHAPE, COMPRISING: AN ELONGATE BASE; A DIE MOUNTED ON AN INTERMEDIATE PORTION OF SAID BASE AND HAVING AN UPWARDLY DIRECTED CONVEX FORMING FACE; A STRETCHING COMPLEX MOUNTED ON EACH END PORTION OF SAID BASE AT OPPOSED SIDES OF SAID DIE AND ADAPTED TO GRIP OPPOSED MARGINAL EDGES OF A METAL BLANK TO STRETCH AND FORM IT ON THE FORMING FACE OF SAID DIE; EACH COMPLEX INCLUDING A PLURALITY OF DISCRETE, INDEPENDENT, ELONGATE STRETCH UNITS; EACH STRETCH UNIT INCLUDING AN INDEPENDENT ANCHOR, A JACK DEVICE PIVOTALLY CONNECTED AT ONE END TO SAID ANCHOR FOR VERTICAL AND HORIZONTAL SWINGING, AND A GRIPPER PIVOTALLY CONNECTED TO THE OTHER END OF THE JACK DEVICE FOR ROTATION AND LATERAL SWINGING AND PROVIDED WITH A JAW TO GRIP A PORTION OF THE MARGIN OF A METAL BLANK; A PLURALITY OF ELONGATE, SUBSTANTIALLY PARALLEL TRACK MEANS AT EACH END OF SAID BASE AND EXTEINDING GENERALY LONGITUDINALLY OF THE BASE; SAID ANCHORS HAVING PORTIONS SLIDABLY INTERENGAGING WITH SAID TRACK MEANS; AND SEP- 